I think that one area that interests me is the idea of using the hyphen. It seems like a tricky thing to use, and I feel like I probably use it incorrectly myself about half the time I do use it.

http://owl.english.purdue.edu/owl/resource/576/01/

Another great minilesson topic could be on when you should underline, italicize, or put quotes around a particular work.

http://owl.english.purdue.edu/owl/resource/560/02/